1. Field of the Invention
The present invention is directed to methods of decoding phase shift keyed (PSK) communications signals. More specifically, but without limitation thereto, the present invention is directed to a method of calculating an error probability of a received 8-phase shift keyed code.
2. Description of the Prior Art
Phase shift keyed signals (PSK) are often used in systems for communicating data over a distance. A phase shift keyed signal consists of a carrier that is modulated in phase increments, for example, 90 degree increments in quadrature, or 4-PSK, 45 degree increments in 8-PSK, and 22.5 degree increments in 16-PSK. Each phase increment corresponds to a code symbol in the phase shift keyed signal. A complete set of code symbols for modulating a phase shift keyed signal is called a trellis modulation code (TCM). A PSK communications system transmitter typically includes a source of the carrier signal and a PSK modulator responsive to an input binary signal for modulating the carrier signal. The PSK communications system receiver typically includes a phase demodulator, or decoder, suitable for extracting instantaneous phase shift information from the carrier signal and recovering the data represented by the phase shift information. A suitable decoder for demodulating the phase modulation is a Viterbi decoder, in which the accuracy of the demodulation is dependent on the electrical noise that may be received along with the modulated carrier signal.